Remote microphone/keypad assemblies such as those used with radio communication equipment (e.g., two-way radios, etc.) are susceptible to radio frequency interference (RFI) as well as electromagnetic interference (EMI). RFI and EMI are found in portable radio applications and especially in mobile radio applications where a mobile radio is mounted in the trunk of a vehicle and the microphone/keypad assembly is wired through the car into the passenger compartment. In such vehicular installations, the push-to-talk switch (PTT switch), keypad and microphone signals are very susceptible to interference.
Another problem encountered in vehicular installations is the voltage drop caused by the extended cable lengths between the microphone assembly and the communication device. Usually, the keypad and PTT signals generated at the remote microphone/keypad assembly are "read" by an analog-to-digital (A/D) converter circuit in the mobile radio which is looking for a particular voltage level corresponding to the button presses. When the extended cable lengths are added, the voltage windows detected by the A/D converter are altered due to the voltage drop caused by the cable's resistance. Also, such long cable lengths act as antennas which increase the possibility of picking up unwanted noise by the cable connecting the remote microphone/keypad assembly with the radio.
Another problem associated with current remote microphone keypad assemblies is that in order to illuminate the keypad, most present day keypad assemblies use an elastomer membrane keypad, a lightpipe in back of the elastomeric membrane, and a light source such as a small incandescent light bulb all located in the remote microphone/keypad assembly. The problem presented by having such a design is that it is required to route supply voltage and ground potential lines through the cable connecting the remote microphone/keypad to the radio, which are susceptible to line losses and interference.
Finally, in applications where an illuminatable keypad or switch is required, present keypad or switch designs require one set of circuitry be used to illuminate the keypad assembly or switch, and another set of circuitry be used to decode the keypad/switch in order to determine when the keypad or switch have been activated. These current designs not only increase the chances of interference as explained previously, but increase the cost and size of the keypad assembly. A need thus exists for an illuminated switch assembly which can provide for immunity from RFI/EMI interference, voltage drops, as well be able to use the circuitry used in illuminating the keypad to decode the key(s) when they are activated.